In known spectrum analyzers, the input mixer requires hand adjustment or uses specially selected mixer diodes to reduce local oscillator ("LO") to intermediate frequency ("IF") leakage. This leakage typically limits the low frequency performance of the spectrum analyzer and introduces residual undesired signals. This mechanical balancing is expensive and does not fully reduce second harmonic distortion products.
One known circuit to reduce LO-IF leakage in a spectrum analyzer is shown in FIG. 1. The LO nulling circuit has a phase quadrature ("IQ") modulator subsystem which generates a signal equal in amplitude and opposite in phase to the undesired LO leakage signal. This signal is coupled into the IF subsystem to cancel the leakage. As the mixer for the radio frequency ("RF") input signal and the LO signal is not actually balanced, second order harmonic distortion products are not significantly reduced. As the nulling circuit requires many RF components external to the mixer itself, this circuit is simply too expensive for a low cost spectrum analyzer.